Transconductance is often an important measure of the performance of an electronic circuit, affecting many performance parameters including, but not limited to, bandwidth, gain and noise. Transconductance is an expression of the performance of certain electronic circuits, and traditionally refers to the ratio of output current to input voltage of a particular circuit, or mutual conductance. The term “transconductance” refers to herein as the control of an output current as a result of an input voltage, and is given by the expression:
                                          g            m                    =                                    Δ              ⁢                                                          ⁢                              I                out                                                    Δ              ⁢                                                          ⁢                              V                                  i                  ⁢                                                                          ⁢                  n                                                                    ,                            (        1        )            
or more generally as,
                                          g            m                    =                                    I              out                                      V                              i                ⁢                                                                  ⁢                n                                                    ,                            (        2        )            
where gm is the transconductance with units of siemens, Iout is the output current and Vin is the input voltage. In general, the larger the transconductance figure for a device or circuit, the greater the gain (amplification) it is capable of delivering, when all other factors are held constant.
In integrated circuits, it may be important for the transconductance of an electronic circuit to remain constant over one or more operating parameters as well as processing variation. The stability or robustness of transconductance of transistors may be an important design parameter, as it may be affected by many operation and processing conditions, such as temperature, carrier mobility, supply voltage, etc. A conventional solution to the problem of achieving constant transconductance is to use the conductance of a stable and precise external (e.g., off-chip) resistor. In the context of an amplifier, for example, a constant reference voltage provided off-chip may be placed across an external resistor to achieve a substantially constant bias current for the amplifier.